equilibrium position
Imagine a seesaw that has come to rest. It might balance level in the middle, or it might settle tilted strongly toward one end. The equilibrium position is which way the chemical 'seesaw' settles — how the final mixture is shared between reactants and products. Two reactions can both be at equilibrium yet sit in very different places: one almost all product, the other almost all reactant.
Concretely, the equilibrium position is the particular set of concentrations (or pressures) present when a reaction reaches equilibrium under given conditions. We speak of it lying 'to the right' when products dominate and 'to the left' when reactants dominate. Crucially, the equilibrium position can be moved — by adding or removing a substance, changing pressure, or changing temperature — even when the underlying reaction is the same.
The key distinction to hold onto is the difference between the equilibrium position and the equilibrium constant K. K is a fixed number set only by temperature. The position is the actual mixture, and it can shift to many values that are all consistent with the same K. So changing concentration or pressure moves the position without changing K; only changing temperature changes K (and the position with it).
Dissolving more reactant into a flask already at equilibrium pushes the equilibrium position to the right, making more product, until a new balance is reached. The mixture is different, but the equilibrium constant K is exactly what it was before.
The position can move; K stays put unless temperature changes.
Position and constant are different things. A shift in the equilibrium position does not by itself mean K changed — usually it has not. Watch for the word 'temperature', the only lever that moves K.